Currently, flat panel displays using techniques for e-paper displays (EPDs), plasma displays (PDPs), liquid crystal displays (LCDs), and organic light emitting displays (OLEDs) are being variously utilized in TV sets, mobile phones, monitors, e-books, mobile devices, etc.
In the next generation of technology, however, flexible displays which are easily portable and conveniently usable regardless of time and place are expected to be widely used in electronic devices such as mobile phones, portable terminals, laptop computers, etc.
A substrate for a flexible display has to be mechanically flexible so that the flexible display is efficiently bendable, rollable or foldable. The flexible substrate may include a very thin glass plate, a thin stainless steel plate, a plastic film, etc. Among them a plastic film is particularly useful.
Moreover, glass is currently commonly used as a cover window for flat panel displays such as TV sets, monitors, and mobile phones. Glass is superior in heat resistance, optical transmittance and mechanical strength but is heavy and brittle. In order to replace glass, a colorless transparent plastic film having superior thermal and optical properties is being studied in the related art.
Although a plastic film substrate has much higher mechanical flexibility than a glass substrate, it has inferior tensile strength. When glass fabric is impregnated in a plastic film, tensile strength of a plastic film, such as fiber reinforced plastic (FRP), may be enhanced as high as that of tempered glass. It has been widely utilized, as disclosed in Korean Patent Application Publication No. 10-2010-0118220 (Title: Flexible substrate for display panel and manufacturing method of the same), Korean Patent Application Publication No. 10-2010-0118222 (Title: A flexible substrate for display panel and a method for manufacturing the same), Korean Patent Application Publication No. 10-2012-0027632 (Title: Fabrication method of flexible device), and Korean Patent Application Publication No. 10-2011-0055425 (Title: Polymer-organic nano-fiber composite having superior thermal expansion property and light-transmission and transparent composite film).
A polyimide film has high mechanical flexibility and is superior in heat resistance, wear resistance, insulating properties, chemical resistance and mechanical strength. This polyimide film has been variously utilized in electronic components, such as insulating films, flexible cables or printed circuit boards (PCBs). A typical polyimide film has dark brown color and thus is not being used in display substrates, despite its superior physical properties as described above. Recently, colorless transparent polyimide is being developed and can be used as a display substrate.
In order to manufacture a display device, a thin film transistor (TFT) should be provided on a substrate to control switching and luminance of individual pixels. Currently available is TFT using amorphous silicon, polysilicon, oxides, organic material, etc. In the case of amorphous silicon that exhibits very stable performance, the minimum processing temperature necessary for deposition and thermal treatment is about 230° C. When a plastic substrate is subjected to a TFT thin film process at 230° C. and then cooled to room temperature, the TFT thin film may be stripped off from the plastic substrate due to a difference in the coefficient of thermal expansion (CTE) between the plastic substrate and the TFT thin film material. To prevent this, the CTE of the substrate should be equal to or less than 10 Ppm (part per million)/° C.
Colorless transparent polyimide (CPI) has a CTE of at least 50 ppm/° C. However, when glass fabric having a CTE of about 5 ppm/° C. is incorporated in the colorless transparent polyimide film, the CTE of the colorless transparent polyimide film having impregnated glass fabric may be lowered to about 10 ppm/° C.
In the course of forming a colorless transparent polyimide film having impregnated glass fabric using a solution process, the surface of the film may be inevitably roughened to the level of tens of nm˜1 μm for the following reasons.
When glass fabric is placed on a glass plate and then impregnated with a polyamic acid solution, the surface of the polyamic acid solution may become completely flat under the force of gravity. However, because the woven surface of the glass fabric is not flat, the thickness from the surface of the glass fabric to the surface of the polyamic acid solution may vary depending on the position.
Specifically, the polyamic acid solution at the portion of the glass fabric where glass fibers are over-positioned may become thin, whereas the polyamic acid solution at the portion of the glass fabric where glass fibers are under-positioned may become thick (FIG. 1a). Then, when the solvent is evaporated and polymerization occurs in the course of forming the colorless transparent polyimide film, a small degree of shrinkage occurs at the portion where the solution is thin and a large degree of shrinkage takes place at the portion where the solution is thick. Accordingly, the surface of the formed colorless transparent polyimide film has irregularities similar to the surface topography of the glass fabric (FIG. 1b), and the surface roughness thereof ranges from tens of nm to 1 μm depending on the thickness of the glass fabric.
When the surface of the substrate is roughened, even if the refractive index of the colorless transparent polyimide film is matched with that of the glass fabric, surface roughness of the order of tens of nm˜1 μm may cause scattering of light, thus drastically reducing optical transmittance and transparency of the substrate and also making it difficult to perform a TFT process necessary for manufacturing a display device.
To increase optical transmittance and transparency of the substrate and to make a TFT process to be feasible on the substrate, the substrate should have a surface roughness of the order of 1 nm.
Because a typical thin-film coating used to flatten the surface of a display substrate has a thickness of a few nm, the surface of the film having a roughness of the order of tens of nm˜1 μm is difficult to be flattened so as to attain a surface roughness of about 1 nm by the above thin-film coating.
Therefore, in order for the surface of the colorless transparent polyimide film having impregnated glass fabric to be flattened to a surface roughness of the order of 1 nm using the thin-film coating, the surface roughness of the colorless transparent polyimide film having impregnated glass fabric needs to be controlled to the level of ones of nm.